One of the problems of liquid crystal display devices, which have been conventionally used as display screens of, for example, lap-top personal computers or word processors, is that a poor image quality results by such a phenomenon as image persistence or image bleeding when displaying a fast-moving image.
In view of this drawback, WO95/01701 (published date: Jan. 12, 1995) (Tokuhyohei 8-500915), Japanese Publication for Unexamined Patent Publication No. 082019/1989 (published date: Mar. 28, 1989) (Tokukaisho 64-082019), and Japanese Publication for Unexamined Patent Publication No. 202286/1999 (Tokukaihei 11-202286) (publication date: Jul. 30, 1999) disclose providing a certain OFF (dark) period per one frame (one vertical synchronize period) for an emitter of a liquid crystal display device, so as to obtain a desirable image quality in fast-moving images.
However, in this conventional technique, a voltage of a rectangular wave is applied to the emitter of the illuminating section. Such application of a rectangular wave to the emitter causes the following problems.
That is, because the applied waveform is a rectangular wave, the emitter has a short life, which is problematic in actual application. For example, when a voltage of a rectangular wave is applied to a common cold cathode tube of a liquid crystal display device, a current flows abruptly through the cold cathode tube at a rise of emission, whereas the current is suddenly shut down in the cold cathode tube at a fall of emission, which may result in a reverse current flow. Such a current behavior is detrimental to life of the cold cathode tube. Further, since the rectangular wave include a high harmonic component, the problem of electromagnetic radiation is posed.
Further, in the foregoing conventional technique, the emitter of the illuminating section is of a white type. In this case, the emitters contain fluorescent materials of at least three colors, corresponding to three primary colors of light. Thus, the emitters have different response times depending on colors, and phases of emission waveforms become different. This causes the coloring phenomenon on image contours in a display of a fast-moving image in particular, thus lowering display quality.
Further, in the foregoing conventional technique, an ON period and an OFF period of the illuminating section exist in one frame. Here, examining temperature of the cold cathode tube, which is the emitter of the illuminating section, the temperature starts to increase from the time of emission, and starts to decrease from the end of emission. Thus, a cooling/heating cycle of a period of one frame is generated on the cold cathode tube.
Such a cooling/heating cycle damages the cold cathode tube and shortens its life. Further, the cooling/heating cycle creates a large temperature difference between the start of emission, at which the temperature of the cold cathode tube is the lowest, and the end of emission, at which the temperature of the cold cathode tube is the highest. It is therefore difficult to maintain the environmental temperature of the cold cathode tube constant. A failure to maintain a constant environmental temperature of the cold cathode tube results in decrease in temperature itself, and luminance is lowered as a result.
The foregoing explained the case of the cold cathode tube, but the same is true for other emitters, for example, such as a light-emitting diode, an electroluminescence element, a hot cathode tube, a mercury lamp, a halogen lamp, and a laser.
Further, Japanese Publication for Unexamined Patent Publication No. 082019/1989 (Tokukaisho 64-082019), Japanese Publication for Unexamined Patent Publication No. 202285/1999 (Tokukaihei 11-202285) (publication date: Jul. 30, 1999), and 202286/1999 (Tokukaihei 11-202286) (publication date: Jul. 30, 1999) teach providing a plurality of emitting areas for the illuminating section in a scanning direction, and having the emitting areas synchronize with a vertical synchronize signal of the image display device. That is, the emitter is adapted so that it is switched ON immediately after scanning of a display section, and is switched OFF after a certain predetermined time period, so as to obtain a desirable display quality.
The illuminating section has a structure wherein cold cathode tubes, etc., are disposed side by side in a scanning direction, parallel to a scanning line, in a back-light section on a back of a display section, and each cold cathode tube illuminates a liquid crystal which corresponds to a predetermined number of scanning lines.
However, when an image is displayed using the foregoing illuminating section, the following problems are posed. That is, in order to improve display quality of a fast-moving image without deficiencies such as image persistence, it is required to illuminate each emitting area with a sufficiently short pulse time width. However, in the foregoing conventional structure, the light from the plurality of cold cathode tubes reaches a display area other than the display area to be illuminated, for example, such as an adjacent display area. Thus, considering a specific display area in an image panel such as a liquid crystal panel, this display area is illuminated by a plurality of cold cathode tubes. As such is the case, even when the area is to be illuminated with a short pulse time width to improve display quality of a fast-moving image as described above, the pulse time width is essentially increased. Thus, the conventional structure fails to improve display quality with a short pulse time width, and the effect of shortening the pulse width time becomes weak.
Further, in the foregoing conventional technique, in successively scanning the illuminating section of the image display device for lighting, each emitter must have an OFF operation. However, this OFF operation poses the following problems.
(1) By the repeated ON and OFF of the emitter at the frame frequency, the emitter is damaged and the life of the emitter becomes short as a result.
(2) By the presence of the OFF period, display luminance is lowered significantly.